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Testing new foundations for offshore wind turbines

Until now, most support structures for offshore wind turbines rest on piles that have been driven deep into the sand. This construction method causes noise and high technical complexity. An alternative to this are suction buckets. These are steel cylinders open at the bottom and closed at the top, which externally resemble inverted buckets and are sucked into the ground by artificial negative pressure inside. Large-scale tests are currently underway at Testzentrum Tragstrukturen in Hanover in order to optimise the installation of the bodies and investigate possible compressive and tensile loads.

In order to erect wind turbines off the coast in the middle of the sea, large underwater structures are required as supporting foundations. Depending on the design, one to four piles are driven deep into the seabed on which the structures are based so that they are anchored firmly in the ground. These piling works on the open sea require considerable technical effort and cause considerable noise, which especially affects marine mammals. Instead of the previous driven piles, suction buckets provide a low-noise and quickly implementable alternative. These hollow steel cylinders, closed at the top and open at the bottom, are literally sucked into the bottom of the sea by a negative pressure generated by pumps inside. This does not cause a piling noise and does not require any heavy installation equipment for the construction. Depending on the design of the buckets anchored in the ground, monopiles or jacket structures can then be built on them for offshore wind turbines. If the wind energy turbines have to be dismantled one day, the foundations can be completely dismantled. The Fraunhofer Institute for Wind Energy Systems (IWES) is testing the installation of demonstration buckets in the foundation pit at Testzentrum Tragstrukturen in Hanover.

Burying the bucket foundations in the sand

The challenge with this foundation procedure is to precisely dimension and distribute the pressure. It is also important to avoid misalignments. In the experiments, the steel cylinder for the bucket foundation, which has a diameter of 1.4 metres, disappeared into the ground within one hour.

The quick installation provides a considerable advantage: a prepared support structure with bucket can be erected in one day. The test piece is designed for a jacket support structure. Here, a lattice structure – comparable to a high-voltage mast – stands on four foundations in the ground. In practice, suction buckets have a diameter of 6 to 15 metres.

Tulio Quiroz, IWES project manager, explains: “It's extremely important to closely monitor the vacuum to avoid damaging the bottom and tilting the bucket. In offshore installations, four separate chambers are therefore used in the bucket to balance the pressure.” For six months the IWES has been testing differently sized test pieces in Hanover in a test series. The aim of the experiments is to explore how the foundation interacts with the ground, waves and currents. The findings will be made available to industry partners in research projects in order to further optimise the process. Suction buckets are already being used in practice. This low-noise technology can also be used to develop areas with sound restrictions.

Test environment similar to the sea

The foundation pit holds 1,250 m³ of sand, is 10 metres deep and the ground is saturated with water using four wells. These conditions correspond to the characteristic soil conditions in the North Sea. Hydraulic actuators transfer large forces to the head of the test piece to simulate pressure and, in particular, tensile loads. One test example combines cyclical loads and large impulses, which in reality would correspond to continuous operation with individual high waves. Strain, tilt and acceleration sensors record all movements. The pit is a test facility at Testzentrum Tragstrukturen, which is based at Leibniz-Universität Hannover. A detailed description of the test centre can be found in the BINE-Projektinfo brochure “Testing towers and foundations” (05/2015). The German Federal Ministry for Economic Affairs and Energy has funded its construction as part of its energy research initiatives.